Could be brushes with death from savagery, night trolls, and titans. I make quite small worlds, with proportionally small populations, so necros don't tend to show up until about 200 years in. More population will bring that number down.
From what I've seen, you can improve this situation by providing a large number of civs, and ideally, one secret per civ.
An example worldgen, with 23 towers by year 200 in a pocket world:
worldgen
Created in DF v0.47.05.
[WORLD_GEN]
[TITLE:THREE-EVEN_2]
[SEED:MukMOK2A0E6eYoIIaEgw]
[HISTORY_SEED:ksyiMy84iMewmAmSYEWw]
[NAME_SEED:qyCy6UAausyAYq0ggEKU]
[CREATURE_SEED:48uM4Oaa0UsG48aOEwWu]
[DIM:17:17]
[EMBARK_POINTS:0]
[END_YEAR:200]
[BEAST_END_YEAR:200:-1]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:100:300:0:0]
[RAINFALL:0:66:0:0]
[TEMPERATURE:50:50:0:0]
[DRAINAGE:33:100:3200:3200]
[VOLCANISM:0:0:0:0]
[SAVAGERY:0:100:0:0]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[POLE:NONE]
[MINERAL_SCARCITY:100]
[MEGABEAST_CAP:0]
[SEMIMEGABEAST_CAP:0]
[TITAN_NUMBER:0]
[TITAN_ATTACK_TRIGGER:0:0:0]
[DEMON_NUMBER:0]
[NIGHT_TROLL_NUMBER:0]
[BOGEYMAN_NUMBER:0]
[NIGHTMARE_NUMBER:0]
[VAMPIRE_NUMBER:0]
[WEREBEAST_NUMBER:0]
[WEREBEAST_ATTACK_TRIGGER:0:0:0]
[SECRET_NUMBER:90]
[REGIONAL_INTERACTION_NUMBER:0]
[DISTURBANCE_INTERACTION_NUMBER:0]
[EVIL_CLOUD_NUMBER:0]
[EVIL_RAIN_NUMBER:0]
[GENERATE_DIVINE_MATERIALS:0]
[ALLOW_DIVINATION:0]
[ALLOW_DEMONIC_EXPERIMENTS:0]
[ALLOW_NECROMANCER_EXPERIMENTS:0]
[ALLOW_NECROMANCER_LIEUTENANTS:0]
[ALLOW_NECROMANCER_GHOULS:0]
[ALLOW_NECROMANCER_SUMMONS:0]
[GOOD_SQ_COUNTS:0:0:0]
[EVIL_SQ_COUNTS:0:0:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:0]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:0]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:1]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:25]
[CAVERN_LAYER_WATER_MAX:25]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:0]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:10]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:1]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:90]
[TOTAL_CIV_POPULATION:100]
[SITE_CAP:0]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
[PS_EL:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300]
[PS_EL:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300]
[PS_EL:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300]
[PS_EL:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300]
[PS_EL:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300:300]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_EL:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_RF:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_RF:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_RF:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_RF:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_RF:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_RF:10:10:10:10:10:10:10:0:0:0:66:66:66:66:66:66:66]
[PS_SV:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_SV:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_SV:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_SV:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0]
[PS_SV:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
[PS_SV:32:32:32:32:32:32:32:0:0:0:100:100:100:100:100:100:100]
If you remove the seeds from that and generate a few worlds, you'll see typically the first towers appear around year 50.
And there's no goblins, demons, titans, megabeasts, or semi-megabeasts in these worldgens. It's just humans, dwarves, and elves.
Increasing either the site count (at all) and civ count or secret count beyond 100 doesn't appear to have a dramatic effect on how many towers you get in the same amount of time. YMMV.
A lot of people have had this request "
I want necromancers/towers everywhere! " over the years, and this is the most reliable method I know of to make that happen. There are likely better methods, too.
To be clear, the method is: Increase the number of civs, and have the secret count equal the number of civs, so that there is at least one secret available, per civ, if that event should come to pass, randomly. Once that's in place, the remaining challenge then is only to make a large number of civs possible (by providing the required racial starting biomes), but limit the overall population and site count, so worldgen doesn't take forever. At least, that's what works for me.